The invention relates to a cruciform, planar specimen, in particular made of sheet metal, for biaxial materials testing in a region of high strain.
To experimentally detect the nonelastic behavior of, for example, metal materials, it is necessary to determine the generally (high) biaxial strains under biaxial stresses. Homogeneous stressed and strained states in material specimens are required in order to determine the material equalizations of the so-called simple materials in the sense of Noll (W. Noll, "A New Mathematical Theory of Simple Materials," Archive for Rational Machanics and Analysis, 48.1 (1972)). Biaxial stressed states in test specimens made of sheet metal can be realized either through the use of cruciform specimens or thin-walled, tubular specimens under longitudinal force, internal pressure and torsion. An arrangement of the latter kind is, however, useful only with low strains.
Known is a planar cross specimen according to Shiratori/Ikegami (J. Mack. Phys. Solids, 1968, vol. 16, pp. 373 to 394, Pergamon Press, Great Britain) and Kreissig (dissertation Jul. 2, 1982). According to FIG. 1 of the drawings, there is illustrated a cross specimen which is suitable for the purpose of materials testing under biaxial stress in the test device according to Kreissig. Due to the static uncertainty of the known cross specimen 1 (continuum problem), the stresses cannot be determined from forces introduced in the cross specimen 1 with tensile elements 2. In a measurement region 3 provided on the specimen, the center of the cross specimen 1, no homogeneous stress and strain states can be obtained.
A cruciform planar specimen of the aforementioned kind is also known from FIGS. 5a and b of DE 32 25 381. The specimen is suitable for materials testing under biaxial stress, and is weakened in the central region by bilateral synclinal formation in order to ensure that the result of stresses will show in the region of interest in which the test forces overlap. Experiments with suitable cross specimens 1 whose central regions 3, however, are not weakened by bilateral synclinal formation, result in an escape of stress trajectories 4 under uniaxial tensile force F, as shown in FIG. 2 of the drawings. The stress trajectories 4 spread apart so that the stresses in the force direction are not constant over the width. By rerouting the stress trajectories 4, transverse stresses are produced that can lead to undesired bulges of the measurement region 3.
It is also known from DE 36 17 455, when testing materials of areal components by means of a test device for biaxial static and/or dynamic tension and/or pressure load, to provide, in assignment to the sides of the components, force transfer elements. Each force transfer element is connected to several force introducing elements generating individual loads and is arranged over the entire respective component side. With this test device, homogeneous stress and strain states cannot be attained in the areal component.
Finally, known is also a rectangular planar specimen used in a test device according to Rivlin and Saunders for rubber-like materials (Rivlin, R. S., and D. W. Saunders: Large Elastic Deformations of Isotropic Materials, VII, Experiments on the Deformation of Rubber. Phil. Trans. Roy. Soc. Lond. A 243, 251 to 288 (53, 55, 57, 67, 93, 95)). As FIG. 3 of the drawings shows, a force is introduced at the edge of the rectangular planar specimen at discrete points. This method of testing is logical only with materials that are capable of withstanding the stress peaks occurring at the load introducing elements, e.g. rubber-like materials. This method of testing is not suitable for sheet metal.